Splitterless multicarrier modem

ABSTRACT

A modem for use in Digital Subscriber Line communications transmits and receives data over the local subscriber loop in common with voice information over the loop, while avoiding the need for voice/data splitters. The modem responds to disruptions associated with “disturbance events” such as on-hook to off-hook transitions and the like by rapidly switching between pre-stored channel parameter control sets defining communications over the loop under varying conditions. In addition to changing parameter control sets responsive to a disturbance event, the modem may also change transmission power levels and other system parameters such as frequency domain equalizer characteristics. Further, provisions are made for reduced bandwidth communications under selected conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based in part on the following applications filed byone or more of the inventors herein:

U.S. Provisional Patent Application Ser. No. 60/071,071, filed Jan. 16,1998 by Richard Gross and Michael Tzannes and entitled “Dual RateMulticarrier Transmission System In A Splitterless Configuration”;

U.S. Provisional Patent Application Ser. No. 60/072,986, filed Jan. 21,1998 by Richard Gross, Marcos Tzannes and Michael Tzannes and entitled“Dual Rate Multicarrier Transmission system In A SplitterlessConfiguration”.

U.S. Provisional Patent Application Ser. No. 60/072,450, filed Jan. 26,1998 by Richard Gross, Marcos Tzannes and Michael Tzannes and entitled“Multicarrier System With Dynamic Power Levels”.

The disclosures of these applications are incorporated by referenceherein in their entirety.

BACKGROUND OF THE INVENTION

A. Field of the invention

The invention relates to telephone communication systems and, moreparticularly, to telephone communication systems which utilize discretemultitone modulation to transmit data over digital subscriber lines.

B. Prior Art

The public switched telephone network (PSTN) provides the most widelyavailable form of electronic communication for most individuals andbusinesses. Because of its ready availability and the substantial costof providing alternative facilities, it is increasingly being calledupon to accommodate the expanding demands for transmission ofsubstantial amounts of data at high rates. Structured originally toprovide voice communication with its consequent narrow bandwidthrequirements, the PSTN increasingly relies on digital systems to meetthe service demand.

A major limiting factor in the ability to implement high rate digitaltransmission has been the subscriber loop between the telephone centraloffice (CO) and the premises of the subscriber. This loop most commonlycomprises a single pair of twisted wires which are well suited tocarrying low-frequency voice communications for which a bandwidth of 0-4kHz is quite adequate, but which do not readily accommodate broad-bandcommunications (i.e., bandwidths on the order of hundreds of kilohertzor more) without adopting new techniques for communication.

One approach to this problem has been the development of discretemultitone digital subscriber line (DMT DSL) technology and its variant,discrete wavelet multitone digital subscriber line (DWMT DSL)technology. These and other forms of discrete multitone digitalsubscriber line technology (such as ADSL, HDSL, etc.) will commonly bereferred to hereinafter generically as “ADSL technology” or frequentlysimply as “DSL”. The operation of discrete multitone systems, and theirapplication to DSL technology, is discussed more fully in “MulticarrierModulation For Data Transmission: An Idea Whose Time Has Come.”, IEEECommnunications Magazine, May, 1990, pp. 5-14.

In DSL technology, communications over the local subscriber loop betweenthe central office and the subscriber premises is accomplished bymodulating the data to be transmitted onto a multiplicity of discretefrequency carriers which are summed together and then transmitted overthe subscriber loop. Individually, the carriers form discrete,non-overlapping communication subchannels of limited bandwidth;collectively, they form what is effectively a broadband communicationschannel. At the receiver end, the carriers are demodulated and the datarecovered from them.

The data symbols that are transmitted over each subchannel carry anumber of bits that may vary from subchannel to subchannel, dependent onthe signal-to-noise ratio (SNR) of the subchannel. The number of bitsthat can accommodated under specified communication conditions is knownas the “bit allocation” of the subchannel, and is calculated for eachsubchannel in a known manner as a function of the measured SNR of thesubchannel and the bit error rate associated with it.

The SNR of the respective subchannels is determined by transmitting areference signal over the various subchannels and measuring the SNR's ofthe received signals. The loading information is typically calculated atthe receiving or “local” end of the subscriber line (e.g., at thesubscriber premises, in the case of transmission from the centraltelephone office to the subscriber, and at the central office in thecase of transmission from the subscriber premises to the central office)and is communicated to the other (transmitting or “remote”) end so thateach transmitter-receiver pair in communication with each other uses thesame information for communication. The bit allocation information isstored at both ends of the communication pair link for use in definingthe number of bits to be used on the respective subchannels intransmitting data to a particular receiver. Other subchannel parameterssuch as subchannel gains, time and frequency domain equalizercoefficients, and other characteristics may also be stored to aid indefining the subchannel.

Information may, of course, be transmitted in either direction over thesubscriber line. For many applications, such as the delivery of video,internet services, etc. to a subscriber, the required bandwidth fromcentral office to subscriber is many times that of the requiredbandwidth from subscriber to central office. One recently developedservice providing such a capability is based on discrete multitoneasymmetric digital subscriber line (DMT ADSL) technology. In one form ofthis service, up to two hundred and fifty six subchannels, each of4312.5 Hz bandwidth, are devoted to downstream (from central office tosubscriber premises) communications, while up to thirty two subchannels,each also of 4312.5 Hz bandwidth, provide upstream (from subscriberpremises to central office) communications. Communication is by way of“frames” of data and control information. In a presently-used form ofADSL communications, sixty eight data frames and one synchronizationframe form a “superframe” that is repeated throughout the transmission.The data frames carry the data that is to be transmitted; thesynchronization or “sync” frame provides a known bit sequence that isused to synchronize the transmitting and receiving modems and that alsofacilitates determination of transmission subchannel characteristicssuch as signal-to-noise ratio (“SNR”), among others.

Although such systems do in fact provide a significantly increasedbandwidth for data communications, special precautions are required toavoid interference with, and from, ordinary voice communications andassociated signaling that may be taking place over the subscriber lineat the same time that the broadband data is being carried. The signalingactivities commonly include, for example, the transmission of ringingsignals, busy tone, off-hook indications, on-hook indications, dialingsignals, and the like, and the actions commonly accompanying them, e.g.,taking the phone off-hook, replacing it on-hook, dialing, etc. Thesevoice communications and their associated signaling, commonly referredto as “plain old telephone service” or POTS , presently are isolatedfrom the data communications by modulating the data communications ontofrequencies that are higher than those used for POTS ; the datacommunications and POTS signals are thereafter separately retrieved byappropriate demodulation and filtering. The filters which separate thedata communications and the POTS are commonly referred to as “POTSsplitters”.

The voice and data communications must be separated at both the centraloffice and the subscriber premises, and thus POTS splitters must beinstalled at both locations. Installation at the central office isgenerally not a significant problem, since a single modem at the centraloffice can serve a large number of subscribers, and technicians arecommonly available there. Installation at the customer premises is aproblem. Typically, a trained technician must visit the premises ofevery subscriber who wishes to use this technology in order to performthe requisite installation. In connection with this, extensive rewiringmay have to be done, dependent on the desired location of the ADSLdevices. This is expensive and discourages the use of DSL technology ona widespread basis.

DSL systems also experience disturbances from other data services onadjacent phone lines (such as ADSL, HDSL, ISDN, or T1 service). Theseservices may commence after the subject ADSL service is alreadyinitiated and, since DSL for internet access is envisioned as analways-on service, the effect of these disturbances must be amelioratedby the subject ADSL transceiver.

SUMMARY OF THE INVENTION A. OBJECTS OF THE INVENTION

Accordingly, it is an object of the invention to provide an improveddigital subscriber line communication system.

Further, it is an object of the invention to provide a digitalsubscriber line communication system which is compatible with existingvoice communication services and which does not require the use of POTSsplitters.

Another object of the invention is to provide an improved digitalsubscriber line communication system that efficiently handles datacommunications despite random interruptions associated with concurrentcarriage of voice communications or disturbances that arise fromconcurrent data services on adjacent phone lines.

B. SUMMARY DESCRIPTION OF THE INVENTION Splitterless Operation

The invention described herein is directed to enhancing the accuracy andreliability of communications in systems using discrete multitonetechnology (DMT) to communicate data over digital subscriber lines (DSL)in the presence of voice communications and other disturbances. Forsimplicity of reference, the apparatus and method of the presentinvention will hereinafter be referred to collectively simply as amodem. One such modem is typically located at a customer premises suchas a home or business and is “downstream” from a central office withwhich it communicates; the other is typically located at the centraloffice and is “upstream” from the customer premises. Consistent withindustry practice, the modems are often referred to herein as “ATU-R”(“ADSL Transceiver Unit, Remote”, i.e., located at the customerpremises) and “ATU-C” (“ADSL Transceiver Unit, Central Office”). Eachmodem includes a transmitter section for transmitting data and areceiver section for receiving data, and is of the discrete multitonetype, i.e., it transmits data over a multiplicity of subchannels oflimited bandwidth. Typically, the upstream or ATU-C modem transmits datato the downstream or ATU-R modem over a first set of subchannels,commonly the higher-frequency subchannels, and receives data from thedownstream or ATU-R modem over a second, usually smaller, set ofsubchanels, commonly the lower-frequency subchannels.

Heretofore, such modems have required POTS splitters when used on linescarrying both voice and data. In accordance with the present invention,we provide a data modem for use in discrete multitone communicationsystems which carry voice and data communications simultaneously andwhich operate without the special filtering provided by POTS splitters;they are thus “splitterless” modems. In the absence of certaindisturbances, referred to herein as “disturbance events” and discussedmore fully hereinafter, the modem of our invention transmits data at arate determined by the transmission capabilities of the system withoutregard to such disturbances. Preferably, this is the maximum data ratethat can be provided for the particular communications subchannel,subject to predefined constraints such as maximum bit error rate,maximum signal power, etc. that may be imposed by other considerations.On the occurrence of a disturbance event on the communications channel,however, the modem of the present invention detects the event andthereupon modifies the subsequent communication operations. Among otherresponses, the modem changes the bit allocations (and thus possibly thecorresponding bit rate) and the subchannel gains among the subchannels,so as to limit interference with and from voice communication activitiesor to compensate for disturbances from other services or sourcessufficiently close to the subject subscriber line as to coupleinterfering signals into the line. The bit allocations and subchannelgains may be altered for communications in either direction, i.e.,upstream, downstream, or both. Effectively, this matches the subchannelcapacity to the selected data rate so as to ensure that thepre-specified bit error rate is not exceeded. On cessation of thedisturbance event, the system is returned to its initial, high-rate,state.

Disturbance Events

Of particular interest to the present invention are disturbance eventsthat arise from the occurrence of voice communication activities overthe data link concurrent with the transmission of data over the link.These activities comprise the voice communications themselves, oractivities such as signaling associated with such communications,together with the response to such activities, such as taking a phoneoff-hook or placing it on-hook. Disturbance events also include otherdisruptive disturbances such as interference from adjacent phone linescaused, for example, by the presence of other DSL services, ISDNservices, T1 services, etc. The cessation of a disturbance event mayitself also comprise a disturbance event. For example, the change of avoice communications device such as a telephone from “on-hook” to“off-hook” status can seriously disrupt communications at a modem unlesscompensated for as described herein or unless otherwise isolated fromthe modem by means of a POTS splitter as was heretofore done; it is thusa disturbance event that must be dealt with. However, the return of sucha device to “on-hook” status can also significantly change the channelcharacteristics and is therefore also a disturbance event that must bedealt with. The invention described herein efficiently addresses theseand other disturbance events.

Channel Control Parameter Sets

In accordance with the present invention, the change in bit allocationis accomplished rapidly and efficiently by switching between storedparameter sets which contain one or more channel control parameters thatdefine data communications by the modem over the subchannels. Theparameters sets are preferably determined at the time of initializationof the modem and stored in registers or other memory (e.g., RAM or ROM)in the modem itself, but may instead be stored in devices external to,and in communication with, the modem, e.g., in personal computers, ondisk drives etc.

In accordance with one embodiment of this invention, the channel controlparameter sets comprise at least a primary set of channel controlparameters, stored in a primary channel control table, which definescommunications in the absence of voice communication activities or otherdisturbances; and one or a plurality of secondary sets of channelcontrol parameters, stored in a secondary channel control table, thatdefine data communications responsive to one or more disturbance events.When communicating under control of the primary channel control table,the modem is described hereinafter as being in its “primary” state; whencommunicating under control of the secondary channel control table, themodem is described hereinafter as being in its “secondary” state. Themodem is switched between parameter sets in its primary and secondarystates responsive to the occurrence and cessation of disturbance events,as well as among parameter sets in the secondary table responsive to achange from one disturbance event to another. Since the parameter setsare pre-stored and thus need not be exchanged at the time of adisturbance event, the switch is made quickly, limited essentially onlyby the speed with which the disturbance event is detected and signaledto em participating in the communication, typically not more than asecond or so. This greatly reduces the interruption in communicationsthat would otherwise be required by a complete reinitialization of themodems that typically extends over six to ten seconds, and itsassociated exchange channel control parameters.

As noted previously, in DSL communications, information transmissiontypically takes place in both directions, i.e. the upstream or ATU-Cmodem transmits downstream to the ATU-R modem over a first set ofsubchannels, and the downstream or ATU-R modem transmits upstream to theATU-C modem over a second, different, set of subchannels. Thetransmitter and receiver at each modem, accordingly, maintaincorresponding channel tables to be used by them in transmitting data to,and receiving data from, the other modem with which it forms acommunications pair. Certain parameters such as time and frequencydomain equalizer coefficients and echo canceller coefficients are“local” to the receiver with which they are associated, and thus need bemaintained only at that receiver. Other parameters such as bitallocations and channel gains are shared with the other modem with whicha given modem is in communication (the “modem pair”) and thus are storedin both modems, so that during a given communication session, thetransmitter of one modem will use the same set of values of a sharedparameter as the receiver of the other modem, and vice versa.

In particular, in DSL communications, a key parameter is the number ofbits that are to be transmitted over the various subchannels. This isknown as the “bit allocation” for the respective subchannels, and is akey element of the primary and secondary parameter sets. It iscalculated in a known manner for each subchannel based on the channelSNR, the acceptable bit error rate, and the noise margin of thesubchannel. Another important element is the gain for each of thesubchannels, and is thus preferably also included in the primary andsecondary parameter sets. Thus, each receiver stores a primary channelcontrol table and a secondary channel control table, each of whichcontains one or more parameter sets that define the subchannel bitallocations to be used by it and by the transmitter of the other modemin communicating with it, and each transmitter also stores a primarychannel control table and a secondary channel control table, each ofwhich define the subchannel bit allocations and gains to be used by itfor transmission to the other receiver and for reception at thatreceiver. For the closest match to the actual line over which they areto communicate, those portions of the primary and secondary channelcontrol table at each receiver that define the parameters for use intransmitting to the particular receiver are preferably determined at themodem at which the receiver is located (the “local modem”), as describedherein, but it will be understood from the detailed description hereinthat such tables may also be determined in other ways.

As long as communications over the subscriber line are not impaired by adisturbance event, the modems use the primary channel control table todefine communications over the subchannels. When, however, a disturbanceevent occurs, the modem that detects the event (herein designated “thelocal modem”; typically, this will be the subscriber modem, ATU-R,particularly in cases of activation of a voice communications device bythe subscriber) notifies the other modem of the need to change to thesecondary channel control table, and identifies the specific bitallocation set and/or gain set in the secondary table when more than onesuch set exists. The notification procedure is described in more detailhereinafter. Communications thereafter continue in accordance with theappropriate parameter set (i.e., bit allocations, subchannel gains, andpossibly other parameters) from the secondary channel control table.This condition continues until a new disturbance event is detected, atwhich time the modems revert to the primary channel control table (inthe event the disturbance is simply the cessation ofcommunication-disrupting disturbances or interferences) or to adifferent parameter set secondary channel control table (in the eventthat the disturbance event is the occurrence of anothercommunication-disrupting disturbance or interference).

In addition to changes in bit allocation among the subchannels, andchanges in subchannel gains, further changes may also be made in suchcommunication parameters as time domain equalizer coefficients,frequency domain equalizer coefficients, and the like. These parametersmay also be stored in the channel control tables for use in controllingcommunications, or may be stored in separate tables. Additionally,changes in power level (and corresponding changes in bit allocation andother communication parameters) for communications in either theupstream or the downsteam direction, or both, may be made, and sets ofcontrol parameters may be defined on these power levels as well for usein controlling communications. These changes are described in fullerdetail below.

As presently contemplated, each modem on the subscribed side of the DSLline will communicate with a corresponding dedicated modem on thecentral office side. Thus, each central office modem (ATU-C) need storethe primary and secondary tables for a specific subscriber only.However, efficiencies may be achieved whenever it is unnecessary toprovide service to each subscriber at all times. Under thesecircumstances, a central office modem may be shared among two or moresubscribers, and switched among them as called for. In such a case, theATU-C will store or have access to a set of channel control tables foreach subscriber modem it is to service.

Table Initialization

In the preferred embodiment of the invention, the primary and secondarychannel control tables are determined in an initial “training” session(“modem initialization”) in which known data is transmitted by onemodem, measured on reception by the other, and the tables calculatedbased on these measurements. Typically, the training session occurs whenthe modem is first installed at the subscriber premises or at thecentral office, and the procedure thus “particularizes” the modem to theenvironment in which it will operate. This environment includes, inaddition to the subject data modem, one or more voice communicationdevices such as telephone handsets, facsimile machines, and other suchdevices which communicate over a voice frequency subchannel, typicallyin the range 0-4 kHz. A primary channel control table, comprising aparameter set including at least a set of subchannel bit allocations,and preferably also subchannel gains, is calculated with each deviceinactive. A secondary channel control table comprising one or more bitcommunication parameter sets (bit allocations, gains, etc.) iscalculated with each voice communication device activated separately,and/or with groups of devices activated concurrently. The tables sodetermined are then stored at the receiver of one modem and additionallyare communicated to the transmitter of the other modem and stored therefor use by both modems in subsequent communications.

An alternative approach determines the secondary channel control table(including one or more parameter sets comprising the table) bycalculation from the primary channel control table. This is accomplishedmost simply, for example, by taking one or more of the parameters (e.g.,the bit allocation parameter which defines the number of bits to be usedfor communication across the respective subchannels) as a percentage,fixed or varying across the subchannels, of the corresponding primaryparameters; or as determined in accordance with a percentage, fixed orvarying across the subchannels, of the SNR's of the respectivesubchannels; or as determined in accordance with a different bit errorrate than provided for in the primary channel control table; or by othertechniques.

As a specific example, a number of different sets of bit allocations inthe secondary channel control table may be determined as differingpercentages (fixed or varying across the subchannels) of thecorresponding set of bit allocations in the primary channel controltable. Each secondary bit allocation set corresponds to the effectcommonly produced by a particular device or class of devices, e.g., atelephone handset, a facsimile machine, etc., as determined by repeatedmeasurements on such devices, and thus may be taken to represent theexpected effect of that device over a range of communication conditions,e.g., with a particular type of subscriber line wiring, at a given rangefrom the central office, etc. The subchannel gains may also then beadjusted based on the redetermined bit allocations. The bit allocationsand subchannel gains so determined form new secondary parameter setswhich may be used responsive to detection of the disturbance events theycharacterize, and which substitute for determination of the secondarybit allocations and gains on the basis of measurements of the actualdisturbances being compensated for.

Alternatively, the secondary channel control table may be determined byadding a power margin to the calculations for each of the entries of theprimary table of a magnitude sufficient to accommodate the interferencefrom activation of the voice communications device or from otherdisturbances. This has the effect of reducing the constellation size forthe table entries. The margin may be uniform across the table entries,or may vary across them, as may the percentage factor when that approachis used. Multiple secondary bit allocation sets may be defined by thisapproach, each based on a different power margin.

One example of the use of varying margins is in response to changes incrosstalk (capacitively coupled noise due to nearby xDSL users, wherethe “x” indicates the possible varieties of DSL such as ADSL, HDSL,etc.). This crosstalk is, in general, more predictable than signalingevents associated with voice communications. The crosstalk spectrum ofxDSL sources is well characterized: see, for example, the T1.413 ADSLstandard published by the American National Standards Institute. From aprimary channel control table associated with a single fullinitialization, a secondary table comprising a family of bit allocationsets can be calculated, each corresponding to a different crosstalklevel. As the number of xDSL systems (and thus crosstalk levels)changes, the ADSL link can quickly switch to one of these automaticallygenerated sets.

The secondary channel control table in the present invention may also beadapted dynamically, e.g., by performing measurements on the transmittedinformation in each superframe during data communications and monitoringthese measurements to determine when the channel performance hassufficiently changed that a different bit allocation set, and possiblydifferent gain set, should be used. We have found that the SNR providesa readily measurable and reliable indicator of the required bitallocations and gains.

In particular, we have found that measurements of the SNR levels acrossa number of the subchannels during a given communications condition orstate provides a “fingerprint” which may reliably be used to quicklyselect a parameter set, such as the set of bit allocations or the set ofgains, for use in subsequent communications during that state. Thesemeasurements may be made, for example, on the sync frame that occurs ineach superframe or, more generally, during the transmission of referenceframes. When the SNR's change by more than a defined amount duringcommunications, the modem at which the measurement is made searches thestored parameter sets for a set whose SNRs on the correspondingsubchannels is closest to the measured SNRs, and selects that set foruse in subsequent communications. If no parameter set is found withindefined limits, the system may be switched to a default state, or acomplete reinitialization may be called for, corresponding to a definedpattern of SNR's across some or all of the subchannels, should be used.SNR measurements may also be made on the data carrying signalsthemselves, i.e., a decision-directed SNR measurement.

Instead of using a multiplicity of secondary subchannel controlparameter sets as described above, a simplified approach may constructand use a single secondary set based on a composite of the bitallocation or other characteristics of the individual devices. In oneembodiment, the composite is formed by selecting, for each subchannel,the minimum bit allocation exhibited by any device for that subchannel,or the most severe characteristic of any other disturbances, thusforming a single “worst case” set that may be used when any device isactivated, regardless of the specific device or disturbance actuallypresent. Or it may be determined as the actual or calculated capacity ofthe line when all devices are actually or theoretically actuatedsimultaneously, or all disturbances are present, or both concurrently.Bit allocations sets may also be determined for combinations of subsetsof such devices and disturbances. A similar approach may be used tohandle the situation where several devices are activated at the sametime, and the effects of other disturbances such as cross talk, etc. mayalso be incorporated into a composite set.

A particular parameter set of the secondary channel control tableremains in use for the duration of the session in which the voice deviceis active or until another change of state occurs, e.g., a further voicedevice is activated or some other disturbance takes place. When thisoccurs, the local modem renews its identification procedure to enabledetermination of the appropriate parameter set for the new conditions.At the end of the session in which the voice device is active, thedevice returns to inactive (i.e., “on-hook”) status and the systemreverts to its original (“on-hook”) status in which the primary channelcontrol table once again is used for communications between the centraloffice and the subscriber.

Switching the subchannel parameter sets in accordance with the presentinvention is extremely fast. It can be accomplished in an interval asshort as several frames, and thus avoids the lengthy (e.g., severalsecond) delay that would otherwise accompany determination,communication, and switching of newly-determined sets. Further, itavoids communicating new parameter sets at a time when communicationshave been impaired and error rates are high. Thus, it minimizesdisruption to the communication process occasioned by disturbanceevents.

Detecting Disturbance Events

During subsequent data communications, identification of the device thatis activated is achieved in one of a number of ways. In one embodimentof the invention, a specific activation signal is transmitted from thedevice to the modem on the same side of the subscriber line as thedevice (referred to herein as “the local modem”) on activation of thedevice. This signal may be transmitted over the communications line towhich the device and the local modem are connected or it may be sentover a dedicated connection between the device and the local modem.

In the preferred embodiment of the invention, the local modem monitorsthe subscriber line to which it and the device are connected and detectsa change in line characteristics when the device is activated. Forexample, the signal to noise ratio (SNR) of the various subchannels canquickly be measured and can be used to identify the particular devicethat is activated. During multiple sets of initializations,corresponding to multiple communication conditions caused by the devicesor by other interferences, the SNR measure for each subchannel isdetermined for each of the conditions to be tracked (i.e., no devicesactivated, devices activated separately, two or more devices activatedconcurrently, adjacent channel interference, etc.) and the measuresstored, along with identification of the particular parameter set orsets with which they are associated. When a device is activated, the SNRmeasurements are used to quickly identify the particular device ordevices that have been activated, and the local modem can thereafterswitch to the appropriate secondary table.

Disturbance events may also be detected in accordance with the presentinvention by monitoring selected transmission characteristics that aredependent on these events. These may comprise, in addition to anycharacteristic SNR accompanying them, such measures as errors in thecyclic redundancy code (CRC) that accompanies transmissions and changesin the error rate of this code; changes in the amplitude, frequency orphase of a pilot tone on the subchannels; or other such indicia. Forwarderror correction code (FEC) is typically used in ADSL transceivers, andchanges in the error rate characteristics of this code, such as how manyerrors have occurred, how many have been corrected, how many areuncorrected, and the like, can be particularly useful in detectingdisturbance events.

In monitoring these characteristics, we distinguish between changescaused by momentary or transient events such as lightning or other suchburst noise disturbances, and those associated with disturbance events,the latter continuing for a significant interval (e.g., on the order ofseconds or more). In particular, in embodiments that monitor CRC errorsor error rates in accordance with the present invention, a switch fromone parameter set to another is provided when the errors extend over anumber of frames or when the error rate changes by a defined amount fora time greater than a defined minimum For example, on the occurrence ofan off-hook event, a severe form of disturbance data communications overa subscriber loop, the number of CRC errors suddenly increases andremains at an increased level until it is dealt with. This isdistinguished from the occurrence of a transient disturbance such as alightning strike which causes a momentary increase in CRC errors thatdoes not persist as long as the system has not lost synchronization.

Thus, in accordance with the present invention, the detection of aninitial change in the CRC error rate over a number of frames in excessof a defined threshold is one example of the detection of a disturbanceevent that will result in switching parameter sets. Similar proceduresmay be undertaken in response to measurement of the signal-to-noiseratio of the subchannel in order to detect a disturbance event based onthis characteristic. The decision as to whether a disturbance event hasoccurred may be based on measurements on a single subchannel; on amultiplicity of subchannels (e.g., the decision to switch parameter setswill be made when more than a defined number of subchannels detect adisturbance event); or the like.

An alternative technique for detecting a disturbance event in accordancewith the present invention is the use of a monitor signal, e.g., a pilottone whose amplitude, frequency, phase or other characteristic ismonitored during data transmission. A sudden change in one or more ofthe monitored characteristics from one frame to another, followed by asmaller or no change in subsequent frames, indicates a disturbance eventto which the modem should respond. The monitor signal may comprise adedicated signal carried by one of the subchannels; a signal carried ona separate control subchannel; a disturbance event itself (e.g., ringingtone, dial tone presence, or other common telephone signals); or othersignals.

Communicating The Occurrence of Disturbance Events

After a disturbance event is detected and the appropriate parameter setcorresponding to the event is identified, the identification iscommunicated to the remote modem by means of a selection signal toenable it also to switch to the corresponding parameter set in thesecondary table. The selection signal may be in the form of a messagetransmitted over one or more subchannels or using a predeterminedprotocol for an embedded operations channel, or it may comprise one ormore tones that identify the particular parameter set. ADSL systems usea “guard band” of several subchannels between the sets of subchannelsused for upstream and downstream transmission. This guard band may beused to transmit the selection tone or tones. In cases where there isonly a single parameter set to be designated, the selection signal maycomprise a simple flag (an element that has only two states, i.e.,on/off, present/absent, etc.) that is sent to the remote modem to selectthe set.

In a further embodiment of the invention, use is made of the framecounters at the ATU-R and ATU-C modems that are commonly provided in DSLsystems. On detecting a disturbance event, the ATU-R modem notifies theATU-C modem of the event and specifies a frame at which the change inparameter set, or change in power level and any accompanying change inother parameters, is to take place. The specification may be direct(i.e., the notification specifies a particular frame number at which thechange to the secondary table is to be made) or indirect (i.e., onreceipt of the notification, the change to the secondary table is madeat one of a predetermined number of frames, e.g., the next frame numberending in “0”, or in “00”, etc., or the nth frame after receiving thenotification, where n is some number greater than 0). On reaching thedesignated frame, both modems (i.e., ATU-R and ATU-C) switch to the newbit allocation set, power level, and other designated parameters.

Alternatively, on detection of a disturbance event, the modems perform a“fast retrain” in order to characterize communications under the newoperating conditions and determine a power and/or bit allocation set tobe used for the communications. A fast retrain performs only a limitedsubset of the full initialization procedures, e.g., bit allocation andsubchannel gain determination,. The retraining modem (typically themodem local to the disturbance initiating the retraining) then comparesthe newly determined parameter set with previously stored sets. If thenewly-determined set is the same as a previously stored set, a message,flag, or tone is communicated by one modem to the other to designatewhich of the stored secondary allocation sets is to be used. Otherwise,the newly determined set is used for communications. In the latterevent, it must be communicated to the other modem in the communicationpair, and communications may be interrupted while this occurs.Nonetheless, on cessation of the event which necessitated a change inparameter sets, the system may simply revert to the primary parameterset, without need for recommunication of that set and thus withoutfurther interrupting communications. With proper care in initialization,in most cases a sufficient array of parameter sets may be defined andexchanged at the outset as to avoid the need for subsequentreinitialization in response to most disturbances.

Changing Power Levels

In addition to changing one or more parameter sets in the modem inresponse to a disturbance event, in accordance with the preferredembodiment of the present invention we also preferably change thecommunications power level in either the upstream or the downstreamdirection, or both, in order to farther enhance reliable communications.Typically, the change is a reduction in the power level in the upstreamdirection so as to minimize interference with the voice communications,as well as to reduce echo into the downstream signal, and it will be sodescribed herein. However, it should be understood that there will besome occasions when an increase in power level is called for, such aswhen interference from adjacent data services requires a higher powerlevel in order to maintain a desired data rate or bit error level, andsuch a change is accommodated by the present invention in the samemanner as that of a decrease. Further, a change in downstream powerlevel may be called for when line conditions change to such an extentthat excessive power would otherwise be fed into the downstream channelfrom the upstream modem

In theory, and in a perfectly linear system, upstream communicationsactivities should have no effect on concurrent voice communicationssince the two activities occur in separate, non-overlapping frequencybands. However, the telephone system in fact is not a linear system, andnonlinearities in the system can and do inject image signals from theupstream subchannel into the voice subchannel, and possibly into thedownstream subchannel as well (i.e., echo), thus producing detectableinterference. In accordance with another aspect of the presentinvention, this effect is reduced below the level of objection byreducing the upstream,power level (the power level at which thesubscriber or downstream modem transmits to the central office orupstream modem) by a given amount or factor when conditions dictate,e.g., when a voice communications device is off-hook and leakage fromthe data communications being conducted interferes with the voicecommunications.

The amount of power reduction may be set in advance. For example, wehave found that a nine db reduction in this power (relative to thattypically used in ADSL applications using splitters to separate the dataand POTS signals) is sufficient in most cases of common interest; underthese circumstances, the system operates in one of two alternative powerlevels at all times. Alternatively, the downstream modem may select oneof several different power levels for use, based on the communicationconditions prevailing at the time resultant from the disturbance event.For example, the downstream modem may be activated to send a test signalinto one or more upstream subchannels and to monitor the leakage (i.e.,the echo) of this signal into one or more downstream subchannels asdetermined, for example, by the SNRs on these subchannels; the powerlevel at which the downstream modem transmits upstream may then beadjusted accordingly in order to minimize the effects of the echo.Commonly, the downstream transmit power is determined by the ATU-R,since the ATU-R is closest to the cause of the disturbance is event. Inthis event, the ATU-R uses a message, flag, or tone to inform the ATU-Cof the desired power level to be used for transmission. In either case,at the end of a session, the power level reverts to that used in the“on-hook” state.

In selecting the desired power level, the transmitting modem signals thereceiving modem in the communications-pair of the desired change(including the designation of a particular power level from amongseveral power levels, where appropriate), and thereafter implements thechange, including switching to a new parameter set associated with thatpower level. In another embodiment of the invention, the receiving modemdetects the power level change at the transmitting modem and switches toa parameter set associated with that power level; upstreamcommunications (i.e., from the ATU-R to the ATU-C) are thereafterconducted at the new power level until the disturbance event (e.g.,off-hook condition, etc.) terminates.

While much of the above has been described in terms of a change in powerlevel in the upstream communications from the subscriber,modem to thecentral office modem, it should be noted that a change in power level inthe opposite direction may also sometimes be called for. This may be thecase, for example, on short subscriber loops (e.g., less than a mile),where the reduced line loss consequent on the greater proximity to thecentral office may result in the central office initially transmittingat an excessive power level. In such cases, the central office or ATU-Cmodem performs the role previously performed by the subscriber or ATU-Rmodem, and vice versa, and a change in power level and other parameterson the downstream communications may be performed as described above.Further, it should also be understood that while it is expected that thepower change will most commonly be one that reduces the power level usedto communicate, it may in some cases increase it. This will occur, forexample, when crosstalk from adjacent services requires an increase inpower level of the subject service in order to compensate for thecrosstalk.

Changing Other Parameters

A further important change made in response to detecting a disturbanceevent is a change in the frequency domain equalizers (“FDQ's”)associated with each subchannel. These equalizers compensate for thediffering distortions (e.g., amplitude loss, phase delay, etc.) sufferedby the data during transmission over the subchannel. Typically, they iscomprise finite impulse response filters with complex coefficients. Thecoefficients are set during the “initialization” or “training” phase ofmodem setup. They may subsequently be adjusted based on reference(known) data in reference frames or sync frames transmitted over thecommunication subchannel. In accordance with the present invention,these filters are adjusted responsive to the transmitted reference datawhen a disturbance event is detected. The coefficient updating may beperformed on all subchannels, or selectively on those whose change inerror rates, signal-to-noise ratios, or other error indicia, indicate adisturbance event.

In accordance with one embodiment of the present invention, thecoefficients of the frequency domain equalizers for communications bothin the absence of a disturbance event or disturbance (“primary FDQcoefficients”) and in the presence of such an event or disturbance(“secondary FDQ coefficients”) are computed and stored during theinitialization or training period. Thereafter, these coefficients areswitched responsive to a disturbance event, as is the case with thechannel control tables, and are returned to an initial state on thecessation of such an event.

In accordance with another embodiment of the invention, the FDQcoefficients are recomputed responsive to detection of a disturbanceevent and then used throughout the remainder of the communicationssession in place of the earlier-stored secondary FDQ tables. Therecomputation is accomplished in a short “retrain” session in whichknown reference data is transmitted between the ATU-R and ATU-C . Thereceived data is compared with the known data and the new F)Qcoefficients are determined accordingly. In addition to the frequencydomain equalizer coefficients, time domain equalizer coefficients andecho cancellation coefficients may also be determined and stored. Suchcoefficients are local to the particular receiver, and thus need not becommunicated to the other modem of the communications pair. Accordingly,any such retrain will be extremely fast, and any consequent disruptionto communication limited.

Excessive Disturbances

In some cases a particular device may cause such interference withcommunications that compensation for that device by the methodsdescribed herein is not practical. This may occur, for example, withantiquated telephones or with particularly complex in-home wiring. Insuch a case, it is desirable to minimize the disruption caused by such adevice by inserting a simple in-line filter between the device and thesubscriber line. The filter may comprise, for example, a simple low-passfilter of not more than a cubic inch in volume and a pair of standardconnectors such as RJ11 connectors through which the filter connects tothe device on one side and to the subscriber line on the other. UnlikePOTS splitters, such a connector needs no trained technician to installit, and thus presents no barrier, cost or otherwise, to acceptance ofADSL modems as described herein. Such a device may be detected bymeasuring the nonlinear distortion of the device when it is activated.This is done by monitoring the echo on the line caused by that device.

Reduced Rate Communications

A further improvement in the operation of the modem of the presentinvention resides in confining the bandwidth of the downstreamtransmission to a subset of that normally provided in ADSLcommunications. This reduces the processing demands on both the local(i.e., central office) and remote (subscriber premises) modems, therebyfacilitating the provision of subscriber premises modems at prices moreacceptable to consumer, non-business, use; additionally, it furtherminimizes interference between data transmission and voicecommunications. For example, limiting the number of subchannels used bythe modem to one hundred and twenty eight as opposed to two hundred andfifty six reduces the downstream bandwidth from 1.1 MHz to approximately552 kHz. When the modem is used with modems that normally provide agreater number of subchannels for such communications, the bitallocations and gains for the subchannels above one hundred and twentyeight are preferably nulled, i.e., set to zero.

The invention is preferably operable with modems that do not have thecapabilities described herein, as well, of course, with modems that do.Accordingly, the modem of the present invention identifies itscapabilities, preferably during initialization, preparatory to dataexchange with another modem. In accordance with the preferred embodimentof the invention, this is preferably done by signaling between themodems that are to participate in communications. The signalingidentifies the type of modems in communication and their characteristicsof significance to the communication session. For example, one form ofADSL transceiver uses a reduced number of subchannels (typically, thirtytwo subchannels upstream and one hundred twenty eight subchannelsdownstream) and provides lower bandwidth communications. A modem havingfill ADSL capabilities that encounters a reduced-rate modem can thenadjust its transmission and reception parameters to match thereduced-rate modem. This may be done, for example, by transmission fromone modem to the other of a tone that is; reserved for such purposes.

In particular, in accordance with the present invention, on initiationof communications between a central office modem and a subscriberpremises modem, the modems identify themselves as “full rate” (i.e.,communicating over two hundred and fifty six subchannels) or “reducedrate” (e.g., communicating over some lesser number of subchannels, e.g.,one hundred and twenty eight). The communication may be performed via aflag (two-state, e.g., “on/off”, “present/absent”), a tone or tones, amessage (n-state, n>2), or other form of communication, and may beinitiated at either end of the communication subchannel, i.e., eitherthe central office end or the customer premises end.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a block and line diagram of a conventional digital subscriberline (DSL) system using POTS splitters that is characteristic of theprior art;

FIG. 2 illustrates an illustrative bit allocation and gains table usedin the apparatus of FIG. 1;

FIG. 3 is a block and line diagram of a splitterless DSL system inaccordance with the present invention;

FIG. 4 is a block diagram of a splitterless transceiver in accordancewith the present invention;

FIGS. 5A-5C illustrates channel control tables constructed and used inaccordance with the present invention;

FIG. 6 is a diagram of one form of disturbance event detector inaccordance with the present invention;

FIG. 7 illustrates the use of a frame counter for communicating theswitching decision to the remote modem;

FIG. 8 illustrates the preferred procedure used for performing a fastretrain of the modems in accordance with the present invention;

FIGS. 9A and 9B illustrate the manner in which channel control tablesmay readily be selected in accordance with the present invention; and

FIG. 10 illustrates alternative configuration for interconnection of themodems of the present invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

FIG. 1 shows an ADSL communications system of the type heretofore usedincorporating “splitters” to separate voice and data communicationstransmitted over a telephone line. As there shown, a telephone centraloffice (“CO”) 10 is connected to a remote subscriber 12 (“CP: CustomerPremises”) by a subscriber line or loop 14. Typically, the subscriberline 14 comprises a pair of twisted copper wires; this has been thetraditional medium for carrying voice communications between a telephonesubscriber or customer and the central office. Designed to carry voicecommunications in a bandwidth of approximately 4 kHz (kilohertz), itsuse has been greatly extended by DSL technology.

The central office is, in turn, connected to a digital data network(“DDN”) 16 for sending and receiving digital data, as well as to apublic switched telephone network (“PSTN”) 18 for sending and receivingvoice and other low frequency communications. The digital data networkis connected to the central office through a digital subscriber lineaccess multiplexer (“DSLAM”) 20, while the switched telephone network isconnected to the central office through a local switch bank 22. TheDSLAM 20 (or its equivalent, such as a data enabled switch line card)connects to a POTS “splitter” 24 through an ADSL transceiver unit-central office (“ATU-C”) 26. The local switch 20 also connects to thesplitter.

The splitter 24 separates data and voice (“POTS”) signals received fromthe line 14. At the subscriber end of line 14, a splitter 30 performsthe same function. In particular, the splitter 30 passes the POTSsignals from line 14 to the appropriate devices such as telephonehandsets 31, 32, and passes the digital data signals to an ADSLtransceiver unit-subscriber (“ATU-R”) 34 for application to datautilization devices such as a personal computer (“PC”) 36 and the like.The transceiver 34 may advantageously be incorporated as a card in thePC itself, similarly, the transceiver 26 is commonly implemented as aline card in the multiplexer 20.

In this approach, a communication channel of a given bandwidth isdivided into a multiplicity of subchannels, each a fraction of thesubchannel bandwidth. Data to be transmitted from one transceiver toanother is modulated onto each subchannel in accordance with theinformation-carrying capacity of the particular subchannel. Because ofdiffering signal-to-noise (“SNR”) characteristics of the subchannels,the amount of data loaded onto a subchannel may differ from subchannelto subchannel. Accordingly, a “bit allocation-table” (shown as table 40at transceiver 26 and table 42 at transceiver 34) is maintained at eachtransceiver to define the number of bits that each will transmit on eachsubchannel to the receiver to which it is connected. These tables arecreated during an initialization process in which test signals aretransmitted by each transceiver to the other and the signals received atthe respective transceivers are measured in order to determine themaximum number of bits that can be transmitted from one transceiver tothe other on the particular line. The bit allocation table determined bya particular transceiver is then transmitted over the digital subscriberline 14 to the other transceiver for use by the other transceiver intransmitting data to that particular transceiver or to any similartransceiver connected to the line 14. The transmission must, of course,be done at a time when the line is not subject to disturbances which mayinterfere with communications. This is a significant limitation, andrestricts the utilization of this approach.

Referring now to FIG. 2, a bit allocation table 42 such as is used inthe customer premises equipment is shown in further detail. Table 40,used at the central office, is essentially the same in construction andoperation and will not further be described. Table 42 has two sections,a first section, 42 a, which defines certain communication parameterssuch as bit allocation capacity and subchannel gain parameters thatcharacterize the respective subchannels and which the transmittersection of transceiver 34 will use in transmitting a signal to the othertransceiver (26) with which it is in communication; and a section 42 bthat defines the parameters that the receiver section of transceiver 34will use in receiving a signal transmitted from the other transceiver.Communications take place over a plurality of subchannels, here shown,for purposes of illustration only, as subchannels “9”, “10”, etc. in thetransmitter section, and subchannels “40”, “41”, etc. in the receiversection. In a full-rate ADSL system, there are up to two hundred andfifty six such subchannels, each of bandwidth 4.1 kHz. For example, inone embodiment of the invention, upstream communications (i.e., from thecustomer premises to the central telephone office) are conducted onsubchanels 8 to 29; downstream communications (from the central officeto the customer premises) are conducted on subchannels 32 to 255;subchannels 30 and 31 form a guard band between upstream and downstreamcommunications that may be used for signaling as described hereinafter.

For each subchannel (“SC”) 50, a field 52 defines the number of bits(“B”) that are to be transmitted over that subchannel by the transmitterof a communications or modem pair, and received by the receiver of thatpair, consistent with the prevailing conditions on the subchannel, e.g.,measured signal-to-noise ratio (SNR), desired error rate, etc.; column54 defines the corresponding gains (“G”) of the subchannels. A firstsection, 42 a, of the table specifies the bit allocations and gains thattransceiver 34 will use in transmitting “upstream” to the transceiver26; and a second section, 42 b, specifies the bit allocations and gainsthat transceiver 34 will use in receiving transmissions from thetransceiver 26. Transceiver 26 has a corresponding table 40 which is themirror image of table 42, that is, the bit allocations specified fortransmission by transceiver 34 are the same as those specified forreception by transceiver 26 and correspondingly for reception bytransceiver 34 and transmission by transceiver 26. The table typicallymay also include a field specifying the gain 54 associated with theparticular subchannel.

As noted above, the splitters 24, 30 combine the data and voicecommunications applied to them for transmission and once again separatethese from each other on reception. This is accomplished by means ofhigh pass and low pass filters which separate the low-frequency voicecommunications from the high-frequency data. The need to utilize suchsplitters, however, imposes a severe impediment to the widespreadadoption of DSL technology by the consumer. In particular, theinstallation of a splitter at the subscriber premises requires a trip tothe premises by a trained technician. This can be quite costly, and willdeter many, if not most, consumers from taking advantage of thistechnology. Nor is incorporating splitters in the communications devicesthemselves a viable option, since this not only increases the cost ofsuch devices, but requires either the purchase of all new devices or theretrofit of the older devices, which again requires skilled help toaccomplish. In accordance with the present invention, we eliminate thesplitter at least at the customer premises, thereby enabling adoptionand use of DSL modems by the end user without the intervention oftrained technical personnel. This, however, requires significant changesin the structure and operation of the DSL transceivers or modems, andthe present invention addresses these changes.

In particular, FIG. 3 shows a DSL transmission system in accordance withthe invention in which the composite voice-data signal transmitted fromthe central office to the subscriber premises is passed to both thesubscriber voice equipment 31, 32 and to the data transceiver or modem34′ without the interposition of a splitter at the subscriber premises.In FIG. 3, components that are unchanged from FIG. 1 retain the samenumbering; components that are modified are designated with a primesuperscript. In place of the single table 30 of the transceiver 26 ofFIG. 1, the transceiver 26′ of FIG. 3 contains a primary channel controltable 41 and a secondary channel control table 43. Similarly,transceiver 34′ of FIG. 3 contains a primary channel control table 45and a secondary channel control table 47. It will also be noted that thesubscriber side splitter has been eliminated in FIG. 3: the reason whythis can be done in the present invention will now be described indetail. It will also be noted that the central office splitter 20 inFIG. 1 has been retained in the configuration of FIG. 3: this isoptional, not mandatory. Retaining a splitter at the central office canimprove the performance somewhat at little cost, since only a singleinstallation is required and that at the central office itself wheretechnical personnel are commonly available in any event. Where this isnot the case, it may be eliminated there also.

Turning now to FIG. 4, the transceiver or modem 34′ is shown in greaterdetail; the modem 26′ is essentially the same for present purposes andwill not be separately described. As indicated, modem 34′ comprises atransmitter module 50; a receiver module 52; a control module 54; aprimary channel control table 45; and a secondary channel control table47. The primary channel control table is shown more fully in FIG. 5A.;the secondary channel control table is shown more fully in FIG. 5B.

In FIG. 5A, the primary channel control table 45 has a transmittersection 45 a which stores a primary set of channel control parametersfor use in transmitting to a remote receiver over a DSL line; and areceiver section 45b which stores a primary set of channel controlparameters for use in receiving communications over a DSL line from aremote transmitter. The subchannels to which the parameters apply areshown in column 45 c. The channel control parameters in the transmittersection 45 a include at least a specification of the bit allocations(“B”) 45 d and preferably also the gains (“G”) 45 e to be used on therespective subchannels during transmission. The receiver sectionsimilarly includes specification of the bit allocations and gains, andpreferably also includes specification of the frequency domain equalizercoefficients (“FDQ”) 45 f, time domain equalizer coefficients (“TDEQ”)45 g, and echo canceller coefficients (“FEC”) 45 h, among others.

Collectively, the parameters: bit allocation, gain, frequency domaincoefficient, time domain coefficient, etc. form a parameter set, each ofwhose members are also sets, e.g. the bit allocation set defining theallocation of bits to each of the subchannels, the gain setting setdefining the gains across the subchannels, etc. In accordance with thepreferred embodiment of the present invention, the primary channelcontrol table stores a single parameter set which has at least onemember, i.e., a bit allocation set, and preferably a gain allocation setas well; this parameter set defines the default communicationsconditions to which the system will revert in the absence of disturbanceevents. The secondary channel control table, however, has at least two,and typically more, parameter sets for controlling transmission andreception over the subscriber lines by the respective modems; these setsdefine communications under various disturbance events which change thedefault conditions.

In particular, in FIG. 5B, the secondary channel control table 47comprises a plurality of parameter sets 47 a, 47 b, 47 c, etc., of whichonly three sets are shown for purposes of illustration. Each parameterset includes a transmit portion 47 d and a receive portion 47 e. In eachportion, one or more parameters are specified, e.g., bit allocations 47f and gains 47 g in the transmit portion 47 d, and frequency domaincoefficients 47 h, time domain coefficients 47 i, and echo cancellationcoefficients 47 j in the receive portion 47 e. The actual values of thecoefficients are typically complex numbers and thus they are representedsimply by letters, e.g., “a”, “b”, etc. in the channel control tables ofFIGS. 5A and 5B. Parameter sets 47 b, 47 c, and the remaining parametersets are similarly constructed. As was the case for the primary channelcontrol table, each parameter (e.g., is bit allocation) is itself a setof elements that define communication conditions, at least in part,across the subchannels to which they apply and which they helpcharacterize.

The primary channel control table containing a bit allocation parameterset is generated in the usual manner, i.e., during initialization(typically, a period preceding the transmission of “working data” asopposed to test data), known data is transmitted to, and received from,the remote modem with which the instant modem is in communication underthe conditions which are to comprise the default condition for themodem. Typically, this will be with all disturbing devices inactivated,so that the highest data rate can be achieved, but the actual conditionswill be selected by the user. The data received at each modem is checkedagainst the data known to have been transmitted and the primary channelcontrol parameters such as bit allocation, subchannel gains, and thelike are calculated accordingly. This table is thereafter used as longas the system remains undisturbed by disturbance events which disruptcommunications over the line.

The secondary channel control table may be determined duringinitialization in the same manner as the primary table, but with devicesthat may cause disturbance events actuated in order to redetermine thechannel control parameters required for communications under the newconditions. These devices may be actuated one by one, and a secondaryparameter control set determined for each and stored in the secondarychannel control table; or they may be actuated in groups of two or more,and parameter sets determined accordingly; or various combinations ofsingle and group actuations may be performed and the correspondingparameter sets determined. Secondary parameter sets may similarly bedetermined from actual measurements with interfering sources such asxDSL transmissions in a common binder with the modems in question, andthe resultant sets stored in the secondary table.

Other methods of determination of the secondary table may be employed.For example, one or more secondary parameter sets may be derived fromthe primary table. Thus, the bit allocation on each subchannel in thesecondary table may be taken as a percentage, fixed or varying acrossthe subchannels, of the bit allocation for each subchannel defined inthe primary table. Alternatively, it may be calculated from the samedata as that of the primary table, but using a larger margin; by using apercentage, fixed or varying across the subchannels, of thesignal-to-noise ratio used in calculating the primary table; byproviding for a different bit error rate than provided for in theprimary; or by other techniques, including those described earlier.Portions of the primary and secondary may be recalculated or improvedupon during the communication session, and stored for subsequent use.The calculation or recalculation may be a one-time event or may occurrepeatedly, including periodically, throughout a communication session.

Further, although use of a multiplicity of parameter sets in thesecondary channel control table will generally provide the best match tothe actual channel conditions and thus more nearly approach optimumcommunications conditions, a simplified second table containing a singlecomposite parameter set may also be used. Thus FIG. 5C shows a number ofsets 49 a-49 d of bit allocations for the subchannels 49 e and which mayrepresent a corresponding number of different communication devices orconditions associated with communications over these subchannels. Asingle composite parameter set 49 f may be formed as a function of theparameter sets 49 a-49 d by, for example, selecting, for eachsubchannel, the minimum bit allocation among the sets 49 a-49 d for eachof the subchannels 49 e. Such a set represents a “worst case” conditionfor activation of any of the devices associated with the sets 49 a-49 d.Other worst case parameter sets may be formed, for example, on selectedgroups of devices, thus providing for the case when several devices ordisturbances are operating simultaneously.

In the absence of a disturbance event, the transceivers 26′, 34′ use theprimary channel control tables 41, 45 for communications. Responsive todetection of a disturbance event, however, the transceivers 26′, 34′switch to one of the parameter sets of the secondary channel controltables 43, 47 to continue the communications under the conditionsspecified by the particular parameter table. These conditions mayspecify a diminished bit rate while maintaining the same bit error rateas is provided with the primary channel control table; or may specifythe same bit rate but at a higher bit error rate; or may specify adiminished bit rate at a correspondingly diminished power level ormargin; or other conditions as determined by the specific channelcontrol tables. On termination of the disturbance condition which causedthe switch, the transceivers 26′, 34′ return to use of the primarytables 41, 45.

Typically, the primary tables provide communications at or near thecapacity of the communications channel over line 14. The secondarytables provide communications over the channel at a diminished rate.Switching between the primary and secondary tables (that is, switchingfrom a primary parameter set to a secondary parameter set) in accordancewith the present invention is fast: it can be accomplished in aninterval as short as several frames (each frame being approximately 250microseconds in current ADSL systems), and thus avoids the lengthy delay(e.g., on the order of several seconds) that would otherwise be requiredfor determination, communication over the subscriber line, and switchingof newly-determined bit allocation tables. Further, it avoidscommunication of such tables over the subscriber line at a time whencommunications have been impaired and error rates are therefore high.Thus, utilization of prestored parameter sets in accordance with thepresent invention minimizes disruption to the communication processoccasioned by disturbance events.

The channel control tables are stored in a storage or memory for rapidaccess and retrieval. Preferably, the storage is a random access memory(“RAM”) incorporated into the modem itself, but also comprise such amemory located in other components accessible to the modem, e.g., in astand-alone memory; in a computer such as a personal computer (“PC”); ina disk drive; or in other elements. Further, the storage may includeportions of other forms of memory, such as read only memory (“ROM”).

In addition to accessing the channel control tables 45 and 47, thecontrol module 54 of FIG. 4 preferably also controls formulation of thesecondary control table when this table is calculated on the basis ofthe primary channel control table. Further, the module 54 monitors theSNR on the subscriber line 14 and calculates the primary and secondarychannel control parameter sets when these sets are based on measurementof actual conditions of the line, as will most commonly be the case. Tothis end, the control module is advantageously implemented as a specialpurpose digital computer or “DSP” chip particularized to the functionsdescribed herein. It may, of course, alternatively be implemented as ageneral purpose computer or in other fashion, as will be understood bythose skilled in the art.

In accordance with the present invention, disturbance events on thesubscriber line are distinguished from transient events such aslightning impulses by mean of their consequences. In particular, asignaling event such as an off-hook signal or an on-hook signaltypically causes sufficient disruption as to preclude furthercommunications without reinitialization. The event is accompanied by anerror code indication that persists throughout the disruption; a changein the amplitude and phase of the physical signal carrying the data orof a pilot tone; the application of a substantial voltage to the line;and other indicia. We monitor the subscriber line for the occurrence ofone of more of these characteristics in order to detect the event.

FIG. 6 illustrates one manner of detecting a disturbance event inaccordance with the present invention. A detector 70, which ispreferably included in control module 54, receives signals from line 14and monitors (step 72) the error code (e.g., CRC errors or the FEC errorcount) associated with the signals for occurrence of an errorindication. If no error is detected (step 74), the detector remains inmonitoring mode without further action. If an error is indicated by theerror code, a counter is incremented (step 76) and the count is thencompared with a predefined threshold (step 78). If the count does notexceed the threshold (step 78, “>N?”), the system remains in monitoringmode and continues to accumulate any detected errors. If the countexceeds the threshold (step 78, Y), the detector emits a “disturbanceevent” detection signal (step 80) which causes the transceiver in whichthe detector 70 is located to initiate the process of switching to theappropriate parameter set in the secondary table. The count is reset(line 81) when this occurs.

Instead of monitoring the error code for characteristic behavior (i.e.,repeated error over successive frames), in accordance with the presentinvention one may monitor the amplitude and phase of the physicalsignals transmitting the data over the subchannel or of a pilot tonetransmitted between modems. On the occurrence of a disturbance event,the amplitude and phase of the physical signal undergo significantchange, i.e., the amplitude suddenly decreases and the phase suddenlyshifts to a new value; thereafter, they maintain approximately their newvalues during successive frames. This behavior may be monitored as shownin FIG. 7 in which a monitor 100 monitors, for example, the amplitude ofa data signal or a pilot tone on line 14 and sets a flip-flop 102 to an“active” state (“Q”) on detecting a change in the amplitude of greaterthan a predefined threshold value. Flip-flop 102 enables (input “E”) acounter 104 connected to receive counting pulses from a frame counter106 whenever a new frame is transmitted or received by the modem. Thesecounting pulses are also applied to a threshold counter 108 whichaccumulates the counts applied to it until it reaches a defined countand then applies the resultant count to a comparator 110 where it iscompared with the count in counter 104. If the contents of the counters104 and 108 are equal, comparator 110 provides an output (“Y”) whichcauses the transceiver to initiate the process of switching to theappropriate table. This also resets the counters 104, 108 and theflip-flop 102. These are also reset (input “R”) if the counts ofcounters 104 and 108 do not match (“N” output of comparator 110).

A similar procedure may be used to generate the table-switching signalbased on monitoring the phase change of data signals or pilot tones asnoted above. Further, although the operation of the event detector ofFIG. 8 has been explained largely in terms of hardware, it will beunderstood that it may also readily be implemented in software, or in acombination of hardware and software, as is true of most of the elementsdescribed herein.

Still a further approach to detecting a disturbance event is to monitorthe disturbance event directly. For example, in the case of off-hook oron-hook signals, a 48 volt dc step voltage is applied to the subscriberline. This signal is sufficiently distinct from other signals as to bereadily detectable directly simply by monitoring the line for a stepvoltage of this size and thereafter generating a table-switching signalin response to its detection. Another approach is to monitor the SNR onone or more subchannels by monitoring the “sync” frames. The presence ofa disturbance from data sources on adjacent phone lines manifests itselfas a change in the subchannel SNR. A direct method of monitoringdisturbance events caused by activation or deactivation ofcommunication-disturbing devices is to directly signal between thedevice and the local modem on occurrence of either of these events. Asshown in FIG. 3, for example, signaling lines 35, 37 may be extendeddirectly between the local modem 34′ and its associated devices 31, 32to directly signal a change in these devices, such as their activation(“off hook”) or deactivation (“on hook”).

In addition to changing the control tables in response to a disturbanceevent, it is desirable to decrease the upstream transmit power level inorder to minimize the interference with the voice communications causedby upstream transmissions, as well as to reduce the leakage of thesetransmissions into the downstream signal (“echo”). These interferencesarise from nonlinearities caused by devices such as telephones that arecoupled to the line, especially when the telephones are off-hook. Theamount of power reduction required to render the interferencesacceptable varies from one telephone to the next. In the preferredembodiment of the invention, a probing signal is used to determine therequired decrease in upstream transmit power. In particular, afterdetecting a disturbance event such as activation or deactivation of atelephone or interference from other sources which can disruptcommunications, the transmitter portion of the ATU-R (the “upstreamtransmitter”) transmits a test signal over the subscriber line atvarying power levels and measures the echo at the receiver portion ofthe ATU-R (the “downstream receiver”). The resultant measurement is usedto determine an upstream transmission power level that minimizes echo atthe downstream receiver or that at least renders it acceptable. The newpower level, of course, is typically associated with a corresponding newparameter set in the channel control parameters.

In addition to changing the bit allocation and gain parameters inresponse to a disturbance event, it is generally necessary to change oneor both of the subchannel equalizers, (i.e., the time-domain equalizersor the frequency-domain equalizers), as well as the echo canceller.Appropriate sets of these parameters may be formed in advance in thesame manner as the bit allocations and channel gains (i.e., in apreliminary training session, sending test communications over thesubscriber line with various devices connected to the line activated,measuring the resultant communication conditions, and determining thevarious parameters based on the measurements), and stored in thesecondary channel control table for recall and use as required.Alternatively, they may be redetermined quickly during a retrainingoperation following detection of a disturbance event and withoutexcessively disrupting communications, since these parameters are localto the receiver and thus need not be transmitted to the other modem inthe communications pair.

In particular, in accordance with the preferred embodiment of theinvention, on detecting a disturbance event, the transceivers enter a“fast retrain” phase, as shown in more detail in FIG. 8. A commondisturbance event is taking a telephone off hook or replacing it onhook, and this is commonly detected at the ATU-R. The fast retrainprocess will be illustrated for such an event, although it will beunderstood that it is not limited to this, and that the retrain may beinitiated for any type of disturbance event, and at either end of thecommunication. Thus, on detecting such an event (FIG. 8, event 200), theATU-R notifies the ATU-C (step 202) to enter the fast retrain mode. Thenotification is preferably performed by transmitting a specific tone tothe ATU-C, but may also comprise a message or other form ofcommunication. On receiving this notification (step 204), the ATU-Cawaits notification from the ATU-R of the power levels to be used forsubsequent communications. This includes at least the upstream powerlevel, and may include the downstream power level as well, sincechanging the upstream power level may impact downstream communicationsto some extent. For purposes of completeness, it will be assumed thatboth of these power levels are to be changed, although it will beunderstood that in many cases, only the upstream power level will bechanged.

The new power levels to be used are determined by the ATU-R (step 208),which transmits a channel-probing test signal to the upstreamtransceiver and measures the resultant echo at the downstream receiver;it then sets the upstream power level to minimize the echo into thedownstream signal, and may also set the downstream power level tominimize the effects of leakage of the upstream transmission into thedownstream transmission at the upstream transmitter. The ATU-R thencommunicates (steps 210, 212) to the ATU-C the selected upstream anddownstream transmission levels, e.g., by transmitting to the upstreamtransceiver one or more tones modulated by binary PSK (phase shiftkeying) signals to ensure robust communication of the power levels. Thepower levels may be specified directly (e.g., as “−30 dbm”), orindirectly (e.g., as “level 3” of a predefined group of levels), and thespecification may identify the actual value of the power level, orsimply the change in power level to be effectuated.

The ATU-R (step 214) and ATU-C (step 216) next commence transmission atthe new power levels for purposes of retraining the equalizers and echocancellers. Preferably, the change to the new power levels issynchronized through use of frame counters which are used in DSL systemsto align transmitters and receivers, but the synchronization may beaccomplished by other means (e.g., by transmitting a tone or message orby simply sending a flag) or may be left unsynchronized. Based on thetraining transmission, the ATU-R and ATU-C determine (steps 218, 220)the time and frequency domain equalizer parameters appropriate to thenew power levels, as well as the appropriate echo cancellercoefficients. The determination may include calculations based on thesemeasurements in order to determine the coefficients, or the measurementsmay be used to select a particular set or sets of coefficients from oneor more precalculated sets stored at the ATU-R and ATU-C, respectively.

For example, as was the case with determination of the power levelsresponsive to a disturbance event, the SNRs on various subchannels maybe used to identify a particular device or devices associated with theevent and thus to select an appropriate prestored parameter set storedat the ATU-R and ATU-C, respectively, simply by transmitting to theother modem in the communication pair a message or tone set thatspecifies the number of the parameter set to be used for subsequentcommunications. The SNR measurements thus serve as a “signature” of thedevice or devices associated with the disturbance event, and allow rapididentification of these devices. This approach can significantly reducethe time required to retrain the equalizers and echo cancellers. Andeven if training is required under particular circumstances, thetraining time can be meaningfully reduced by using prestoredcoefficients as the starting point.

To facilitate use of the SNR measurements in retrieving correspondingparameter sets, it is desirable that the various parameter sets asstored be indexed to sets of SNRs, so that one or more parameter setsassociated with particular communication conditions may quickly beidentified and retrieved. One way in which this may be accomplished isshown in FIG. 9A in which the respective parameter sets such as a firstset 250, a second set 252, etc. have, in addition to the subchannel (SC)number 254 and the corresponding bit allocation (BA) and gain (G)entries, a SNR entry 260 characteristic of the parameter set appropriateto a given communication condition, such as “on-hook” (table 250),“off-hook” (table 252), etc. Additional parameter sets such as frequencydomain equalizer coefficients, time domain equalizer coefficients, andecho cancellation coefficients may also be stored in the tables, aswould be appropriate for the receiver portion of the modem; for thetransmitter portion, these coefficients are not applicable and thus arenot stored.

An alternative means of linking the subchannel SNRs and thecorresponding parameter sets is shown in FIG. 9B. As there shown, asimple list structure 270 comprises a parameter set identifier 272, anda multiplicity of SNR measures 274, 276, etc. SNRs for some or all ofthe subchannels may be included. The list may be searched measure formeasure to identify the nearest match to a stored parameter set, andthat set then retrieved for subsequent use. In either FIG. 9A or 9B theparameter set indexed to the SNRs may be a set of multiple parameters,such as bit allocations and gains, among others, of may comprise asingle set such bit allocations only, or gains, only, etc.

The identification of the channel control parameter sets to be used forthe subsequent communications is exchanged between the transceivers(steps 226-232) which then switch to these parameter sets (234, 236) andcommence communications under the new conditions. The message containingthe channel control parameters is preferably modulated in a similarmanner as the “power level” message, i.e., using several modulatingtones with BPSK signaling. The message is therefore short and veryrobust. It is important that it be short so that the fast retrain timeis minimized, since the modem is not transmitting or receiving dataduring this time and its temporary unavailability may thus be verynoticeable, as would be the case, for example, when the modem is beingused for video transmission, or internet access, etc. Similarly, it isimportant that the message transmission be robust, since error-freecommunication during a disturbance event is very difficult, due todecreased SNR, impulse noise from ringing or dialing, or the like. Thus,the provision and utilization of pre-stored parameter sets significantlyenhances the reliability of communications despite the absence of asplitter at at least one of the modems and despite the presence ofdisturbance events concurrent with data communications.

It is expected that the modems described herein will most commonly beused in dedicated pairs, i.e., each subscriber (ATU-R) modem willcommunicate with a dedicated central office (ATU-C) modem. However, incertain cases it may suffice to provide a single master central officemodem to service two or more subscriber modems. The present inventionaccommodates that eventuality as well. Thus, in FIG. 10, a centraloffice modem 280 communicates through a switch 282 with a plurality ofsubscriber modems 284, 286, 288 over subscriber lines 290, 292, 294. Themodems may be located at differing distances from the central office andin different communication environments, and thus the channel controltables of each may be unique among themselves. Accordingly, the centraloffice modem stores a master set 296 of individual channel controlparameter sets 298, 300, 302, etc., one set (both transmit and receive)for each subscriber modem. On initiating communications to a particularsubscriber, the central office modem retrieves the appropriatetransmission parameter set for the subscriber and uses it in thesubsequent communications. Similarly, on initiating communications tothe central office, a given subscriber modem identifies itself to enablethe central office modem to retrieve the appropriate reception parameterset for that subscriber.

CONCLUSION

From the foregoing it will be seen that we have provided an improvedcommunications system for communication over subchannels of limitedbandwidth such as ordinary residential telephone lines. The systemaccommodates both voice and data communications over the linessimultaneously, and eliminates the need for the installation and use of“splitters”, an expense that might otherwise inhibit the adoption anduse of the high communication capacity offered by DSL systems. Thus, itmay be implemented and used as widely as conventional modems are today,but offers significantly greater bandwidth than is currently attainablewith such modems.

What is claimed is:
 1. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel comprising: means for storing a first channel control table for allocating bits to said subchannels during a first communication condition; means for storing a second channel control table for allocating bits to said subchannels during a second communication condition; and means for switching between said tables on the detection of a defined event.
 2. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel comprising: A. means for storing a first channel control table for allocating bits to said subchannels during a first communication condition, wherein said first table specifies the communications capabilities of said modem during normal operation; and B. means for storing a second channel control table for allocating bits to said subchannels during a second communication condition.
 3. A modem according to claim 2 in which the bit allocations of second table are determined by adding noise margins to the determination of the bit allocations of the corresponding subchannels of said first table.
 4. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel comprising: A. means for storing a first channel control table for allocating bits to said subchannels during a first communication condition; and B. means for storing a second channel control table for allocating bits to said subchannels during a second communication condition, wherein said second table specifies the communications capabilities of said modem during diminished operation.
 5. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel comprising: A. means for storing a first channel control table for allocating bits to said subchannels during a first communication condition; and B. means for storing a second channel control table for allocating bits to said subchannels during a second communication condition, wherein said defined event includes signaling events comprising transitions between on-hook and off-hook conditions.
 6. A modem according to claim 5 in which said first table defines communications in the absence of a signaling event.
 7. A modem according to claim 6 in which said second table defines communications responsive to detection of a signaling event.
 8. A modem according to claim 7 in which said switching means switches from said second table to said first table on detection of a signaling event indicative of cessation of a previously-detected signaling event.
 9. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel comprising: A. means for storing a first channel control table for allocating bits to said subchannels during a first communication condition; and B. means for storing a second channel control table for allocating bits to said subchannels during a second communication condition, wherein said first and second tables are determined during an initialization session in which the communication capabilities of said subchannels are determined.
 10. A modem according to claim 9 in which said first table is determined in the absence of interfering signaling conditions.
 11. A modem according to claim 10 in which said second table is determined as a function of said first table.
 12. A modem according to claim 11 in which the bit allocations of said second table are determined as a percentage of the bit allocations of said first table.
 13. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel, comprising: A. means for storing a first channel control table for allocating bits to said subchannels during a first communication condition; and B. means for storing a second channel control table for allocating bits to said subchannels during a second communication condition, wherein said second channel control table is determined responsive to a plurality of signaling events created by a corresponding plurality of event-generating sources, each defining a channel control table specific to the given source, and comprises a composite table formed by selecting, for each subchannel, the minimum bit allocation for the corresponding subchannel of the table associated with each of the plurality of sources.
 14. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel, the improvement comprising: A. means for storing a first channel control table for allocating bits to said subchannels during a first communication condition; and B. means for storing a second channel control table for allocating bits to said subchannels during a second communication condition, wherein said second channel control table is selected from a plurality of tables determined responsive to a plurality of signaling events created by a corresponding plurality of event-generating sources, each defining a channel control table specific to the given source.
 15. A modem according to claim 14 which includes means for selecting one of said plurality of tables for use as said second table in accordance with the source generating an event.
 16. In a modem communicating data over a multiplicity of discrete subchannels, each characterized by a bit allocation parameter defining the allocation of bits over the corresponding subchannel for communication over said subchannel, the improvement comprising: A. means for storing a first channel control table for allocating bits to said subchannels during a first communication condition; B. means for storing a second channel control table for allocating bits to said subchannels during a second communication condition; C. means for redetermining said channel control tables while said modem is in either of said communication conditions; and D. means for communicating a redetermined table to a second modem engaged in communication with said modem.
 17. A modem according to claim 16 in which said communicating means communicates said redetermined table over a dedicated sub-subchannel selected from among said discrete subchannels.
 18. A modem according to claim 16 in which said communicating means further communicates to said second modem information identifying the type of said redetermined table.
 19. A modem for use in asymmetric digital subscriber line communications having both upstream and downstream communication subchannels formed from a plurality of subchannels, said loop adapted to carry both voice and data communications thereon, comprising: means for storing a first table defining data communications between said modem and a second modem connected to said loop during a first communication state; means for storing a second table defining data communications between said modem and said second modem during a second communication state; means for detecting said selected events; means for monitoring a selected characteristic of at least one of said communication subchannels during a plurality of communication intervals; means for determining differences in the selected characteristic over said plurality of intervals; and means for generating a signal initiating switching of said tables when said differences exhibit a defined pattern.
 20. A modem according to claim 19 in which said pattern comprises an initial difference above a first threshold amount followed by at least a subsequent differences less than a second threshold amount.
 21. A modem according to claim 20 in which said first threshold is greater than said second threshold.
 22. A modem according to claim 21 in which said pattern comprises an initial difference above a first threshold amount followed by a plurality of subsequent differences less than a second threshold amount.
 23. A modem according to claim 21 in which said characteristic comprises an error code error.
 24. A modem according to claim 21 in which said characteristic comprises a signal-to-noise ratio.
 25. A modem according to claim 20 in which said selected characteristic is monitored over at least one sub-subchannel.
 26. A modem according to claim 20 in which said selected characteristic is monitored over a plurality of subchannels.
 27. (Amended) A modem according to claim 26 which includes means for averaging the monitored values of said selected characteristic over said subchannels for use in comparing said initial difference to said first threshold.
 28. A modem according to claim 26 which includes means for averaging the monitored values of said selected characteristic over said subchannels for use in comparing said subsequent difference to said second threshold.
 29. A modem according to claim 19 in which said characteristic comprises a parameter of a pilot tone.
 30. A modem according to claim 19 in which said switching means returns said modem to said first communication state on termination of the event causing the switching.
 31. A modem according to claim 19 in which said generating means causes transmission of a switch-control signal over one of said subchannels in response to detection of a selected event.
 32. A modem according to claim 19 in which said generating means causes transmission of a tone in response to detection of a selected event.
 33. A modem for use in asymmetric digital subscriber line communications over a loop having both upstream and downstream communication channels formed from a plurality of subchannels, said loop adapted to carry both voice and data communications thereon, comprising: A. means for storing a first table defining data communications between said modem and a second modem connected to said loop during a first communication state; and B. means for storing a second table defining data communications between said modem and said second modem during a second communication state, specifies wherein said first table establishes a data rate greater than that of said second table.
 34. A modem according to claim 33 in which said tables define the number of bits transmitted over the respective subchannels.
 35. A modem according to claim 34 in which said events comprise signaling events selected from the group comprising off-hook on-hook, ringing, and busy.
 36. A modem for use in asymmetric digital subscriber line communications over a loop having both upstream and downstream communication channels formed from a plurality of subchannels, said loop adapted to carry both voice and data communications thereon, comprising: A. means for storing a first table defining data communications between said modem and a second modem connected to said loop during a first communication state; B. means for storing a second table defining data communications between said modem and said second modem during a second communication state; C. means for emitting into said loop a test signal for probing the return characteristics of transmissions into the loop by said modem; and D. means for limiting the power level of said transmissions in accordance with the measured return characteristics.
 37. A modem according to claim 36 in which said probe comprises a tone at a defined amplitude and frequency and in which the measured return characteristics comprise at least one characteristic selected from the group comprising the amplitude and frequency of the signal returned to said modem in response to emission of said tone.
 38. A modem according to claim 36 in which said probe comprises a plurality of tones at defined amplitudes and frequencies and in which the measured return characteristics comprise at least one characteristic selected from the group comprising the amplitudes and frequencies of the signal returned to said modem in response to emission of said tone.
 39. A modem for use in asymmetric digital subscriber line communications over a loop having both upstream and downstream communication channels formed from a plurality of subchannels, said loop adapted to carry both voice and data communications thereon, comprising: A. means for storing a first table defining data communications between said modem and a second modem connected to said loop during a first communication state; B. means for storing a second table defining data communications between said modem and said second modem during a second communication state; and equalizers for equalizing the transmission characteristics of said subchannels and in which said tables define: (1) coefficients of time domain equalizers, (2) coefficients of frequency domain equalizers or (3) coefficients of digital echo cancellers.
 40. A modem for use in asymmetric digital subscriber line communications over a loop having both upstream and downstream communication channels formed from a plurality of subchannels, said loop adapted to carry both voice and data communications thereon, comprising: A. means for storing a first table defining data communications between said modem and a second modem connected to said loop during a first communication state, wherein said first table is determined during an initialization process in the absence of a selected event; and B. means for storing a second table defining data communications between said modem and said second modem during a second communication state.
 41. A modem according to claim 40 in which said second table is determined during an initialization process in the presence of a selected event.
 42. A modem according to claim 41 in which said second table is redetermined responsive to occurrence of a selected event.
 43. A modem according to claim 42 in which redetermined tables are communicated from a given modem to other modems with which it is in communication during a quiescent state.
 44. A method of transmitting data over a wire line through upstream and downstream channels, respectively, from first and second pluralities of discrete-frequency subchannels, comprising the steps of: A. storing at least first and second parameter sets defining data communications over said channels under at least two different communication conditions; B. selecting a parameter set for use in communications in accordance with the prevailing communication condition, wherein said selecting step includes the step of monitoring communications on said line and transmitting and selecting said parameter set in accordance with said monitoring.
 45. The method of claim 44 in which said monitoring step includes the step of measuring at least one communication indicium on said at least one subchannel.
 46. The method of claim 45 in which said at least one indicium is selected from the group comprising signal to noise ratios, error rates, and the amplitude and frequency of tones.
 47. A method of transmitting data over a wire line through upstream and downstream channels, respectively, from first and second pluralities of discrete-frequency subchannels, comprising the steps of: A. storing at least first and second parameter sets defining data communications over said channels under at least two different communication conditions; B. selecting a parameter set for use in communications in accordance with the prevailing communication condition; and C. transmitting over said line a signal that identifies the parameter set to be selected.
 48. The method of claim 47 in which said signal is transmitted on a subchannel intermediate said upstream and downstream channels.
 49. A method of transmitting data over a wire line through upstream and downstream channels, respectively, from first and second pluralities of discrete-frequency subchannels, comprising the steps of: A. storing at least first and second parameter sets defining data communications over said channels under at least two different communication conditions; B. selecting a parameter set for use in communications in accordance with the prevailing communication condition; and C. receiving over said line a signal that identifies the parameter set to be selected.
 50. The method of claim 49 in which said signal is received on a subchannel intermediate said upstream and downstream channels.
 51. A method of transmitting data over a wire line through upstream and downstream channels, respectively, from first and second pluralities of discrete-frequency subchanels, comprising the steps of: A. storing at least first and second parameter sets defining data communications over said channels under at least two different communication conditions; B. selecting a parameter set for use in communications in accordance with the prevailing communication condition, wherein said first parameter set defines communications over said line in the absence of a disturbance event and said second parameter set defines communications over said line in the presence of a disturbance event.
 52. A method of transmitting data over a wire line through upstream and downstream channels, respectively, from first and second pluralities of discrete-frequency subchannels, comprising the steps of: A. storing at least first and second parameter sets defining data communications over said channels under at least two different communication conditions; B. selecting a parameter set for use in communications in accordance with the prevailing communication condition, wherein said parameter sets include at least one parameter set from the group comprising subchannel bit allocations and subchannel gains.
 53. A method of transmitting data over a wire line through upstream and downstream channels, respectively, from first and second pluralities of discrete-frequency subchannels, comprising the steps of: A. storing at least first and second parameter sets defining data communications over said channels under at least two different communication conditions; B. selecting a parameter set for use in communications in accordance with the prevailing communication condition, wherein said parameter sets include at least one parameter set from the group comprising subchannel frequency domain coefficients, time domain coefficients, and echo cancellation coefficients.
 54. The method of claim 52 in which said parameter sets include a first section for use in transmitting data over said line and a second portion for receiving data over said line. 